Suppression of crosstalk



Aug. 25, 1942. J. o. EDSON SUPPRESSION OF CROSSTALK Filed Aug. 14, 19413 Sheets-Sheet 1 ,INVENTOP J 0. EDSON BY ATTORNEY Aug. 25, 1942. J. o.EDSON SUPPRESSION OF CROSSTALK Filed Aug. 14, 1941 3 Sheets-Sheet 2INVENTOR J 0. E DSON A TTOR/VEY 5, 9 J. OMEDSON Y 2,293,720

- SUPPRESSION 0F CROSSTALK Filed Aug. 14, 1941 s Sheets-Sheet 5 lllllllluvmvron J. 0. EDSON ATTORNEY PatentedAug. 25, 1942 SUPPRESSION orCROSSTALK James 0. Edson, Great Kills, N. Y., assignor to Bell TelephoneLaboratories, Incorporated, New York, N. Y., a corporation of New YorkApplication August 14, 1941, Serial No. 406,799

'JClaims- Cl.17978) I The present invention relates particularly toreduction of crosstalk between multistage amplifiers such as are used intelephone repeaterstations where all the amplifiers receive their spacecurrent supply from a common source.

Suppression of crosstalk between amplifiers arising from the use of acommon battery requires the use of some type of filtering that'willpermit the supply of direct current from the battery to the variousamplifier elements but will not introduce sufiicient impedance n thecommon leads at the signal frequency to produce intolerable amounts ofcrosstalk.

' It is common practice to use a certain amount? of filtering peramplifier per stage, such filtering in each case being kept to a minimumfor reasons of economy, because of the large number of such filtersrequired. I

In accordance with the present invention,

where a number or similar multistage amplifiers are installed at acommon point, the corresponding stages of the amplifiers are grouped forpurscheme in a case where it is desired to use the poses of currentsupply and filtering, thereby filtering in the filters that are commonto;the

reducing the number of filter units required and making economical amore complete degree of -It is found that when the this basis, there isalso an economy'cf space requirements on the amplifier panels.

filtering is done on In order for one circuit to produce noticeablecrosstalk into another, the former must give rise to currents in thelatter of sumcientf magnitude to be disturbing. It the level of thecurrents of the former is higher than those of the latter and ii the twocircuits are coupled by high impedance, the higher level circuit isbound to produce disturbing crosstalk in the circuit of lower level.

If the stages of like levelof each of the several amplifiers are groupedtogether and their space currents supplied through one common filter,

only one such filter is required forall stages of.

the same level. With the required'number of filters thus reduced, onecan afford to make high the inductance of the coil and the capacity-ofthe condenser oieach filter and thus achieve a very low shunt couplingimpedance between all stages of like level and a very high seriesimpedansce between stage groups of unlike level. Since the signalcurrents inrstages oi the same level are of the same magnitude therewill be but little tendency for one tocrosstalk into another. withstages grouped according to level, each :group having its own low-passfilter with a very low cut-ofi, the tendency for a group of higher levelto crosstalk into a group of lower level is also reduced by theseparation effect of the filters. Fig. 1 shows the most generalapplication of the filtering idea applied to the case where each stageis supplied with plate current in the usual Fig. 2 shows a modified formof the filtering combined voltage of plate and filament to provide ahigher voltage to the amplifier than the plate alone can supplyrand Fig.3 shows another modification of this filtering scheme to a case in whichthe available space current battery voltage is correct for the outputstage but must be supplied to the other two stages in series.

Fig. 1 shows schematically a group of amplifiers A1, A: to As, eachamplifier being inserted in the path of a pair of leads which form partof a large group of communication circuits such as a telephone cable.Signal currents on line 5| are. introduced into amplifier A1 throughinput transformer 52 and then pass through the various stages ofamplification 3|,32 and 33 and finally pass out through outputtransformer 53 to output pair 55 thereby being raised to a levelsufficient to withstand the attenuation they will ex- .perience betweenthis and the next repeater point. The electron discharge devices M and42 shown in stages 3| and 32 may be voltage pentodes such as the WesternElectric 3l0-A and the one,

that is, device 43,'in the output stage 33 may be a power pentode suchas the 311-A.

The amplifier A1 of itself forms no part of the present invention; itcan have any number of stages and may or may not have stabilizedfeedback. Its space current may be taken from the output terminals ofany direct current source such as battery P.

In Fig. 1 the battery filter F is composed of several low-pass filtersconnected to the battery I in parallel with each other. Since all thecomponent filters are, alike in configuration and since all stages ofamplification of this amplifier are substantially alike in configurationan explanation of the relationship between section 2| 0! filter F tostage 3| of amplifier A1 will sufiice as typical of all other stages.The various terminals of the filter are connected to their correspondingterminals on all amplifiers as shown. The negative terminal 6 of thebattery P is run to ground [0. Then the path of the direct cur rent forall the first stages, since they are all connected in parallel, is fromthe positive'terminal l of the space current power supply 1? through thefirst stage filter retard coil 2 to the screen grid of tube 4| andthrough retard coil 3 to its plate, thence through the space of tube 4|to its cathode, and suppressor grid, through cathode resistance 4 andthence back to the negative terminal 6 of power supply P. Alternatingcurrent is generated in tube 4| by the variable control action of itssignal grid on the direct current flowing through its space. The path ofthis alternating current around the first stage of the amplifier is thesame as that of the direct current (except that most of it is shuntedfrom bias resistor 4 by the cathode condenser but in passing through thefilter almost all of it is shunted by the first stage filter condenserOnly that portion of the alternating current that passes through thebattery can produce crosstalk in the other stage groups. If the capacityof condenser is made very high, its impedance will be very low and sincethis advantage is further enhanced by the fact that the currents of allfirst stages are of the same level, the voltage drop levels.

Sometimes it is desirable to make available the combined voltage ofplate and filament to supply space current for the tubes, as when it isdesired to increase the power output of repeater amplifiers. In such acase it might be necessary in a repeater ofilce to put a ground on thebattery at a point between its terminals. The circuit shown in Fig. 2provides for that condition. The principle, as far as space currentsupply is concerned, is the same as for Fig. 1, the battery filter Fbeing modified as shown. The cathode of the first tube 4| is maintainedat a negative potential by connecting it to battery through coil 8 whichis bypassed to ground through condenser 9. The junction point betweencondensers 9 and I is brought to the amplifier panel ground. A similarfilter supplies power to the second stage and still another suppliespower to the third stage.

The power supply system of Fig. 2 is connected to all amplifiers asshown. This particular power supply comprises the customary platevoltage source included between terminal I and ground and the filamentvoltage battery included between ground and terminal 6. Thebatteryconnected to the other side of terminal 6 is a C battery, the functionof which will be discussed later. The connections to the amplifiers aresuch that the filament heating battery and plate battery are employed inseries to supply space voltage to the tubes with the filament batterybelow ground potential. When it is desired to operate the filament andplate batteries of a repeater ofilce in series in order to get a higherspace current voltage the direct space current must be returned to thenegative end of the filament battery which is not grounded while at thesame time the alternating current generated in the space of thetube mustbe prevented from passing through the battery. To accomplish thisrequires anotherlow-pass battery filter made up of inductance 9 andcondenser 8, which acts as a filter around the filament battery tosupplement the similar inductance condenser combination 1 and 2 aroundthe plate battery. Thus under the conditions of this circuit the path ofthe direct current for all the first stages, is from the positiveterminal of the space current power supply P through the first stagefilter retard coil 2 to the screen grid of tube 4| and through retardcoil 3 to its plate, thence through the space of tube 4| to its cathodeand suppressor grid, through cathode resistance 4, thence through thefirst stage filter retard coil 8 and back to the negativeterminal 6 ofpower supply P. The path of the alternating current, generated in thetube, around the first stage of the amplifier is the same as that of thedirect current except that a great deal of it is bypassed to ground bycondenser 5 and substantially all the rest is shunted around the spacebattery through condensers 9 and 1. The use of combined filament andplate battery voltage to realize increased space current is disclosedinthe United States patent to J. O. Edson et al. No. 2,191,167, grantedFebruary 20, 1940 to which reference may be made for more detailedinformation.

The additional circuit elements shown in Fig. 2, namely resistance 62,condensers BI and 65 and inductance 64, have to do with the feedbackcircuit of this amplifier and are not related to the invention. Theadditional wire between each section of the filter F and all amplifiersis furnished to connect the mid-points with the amplifier panel ground I2 and the battery ground l0 which is intermediate the battery in orderto get the higher voltage discussed above. Additional wire 14 betweenall amplifiers and filter F is provided to connect the negative terminalof grid bias battery 1| and its associated resistance 1.2 common to thegrid circuits of the output stages of all amplifiers. Condenser 63provides an alternating current shunt circuit around grid bias battery II.

Fig. 3 illustrates an application of the present invention to anamplifier having directly heated filament type cathodes each filamentbeing supplied by a different winding of a filament transformer. Theamplifier receives its space current from a power source having thecorrect space voltage for the output .tube but being equal to the sum ofthe space voltages required for the tubes of the first two stages whichare therefore operated in series, the power source being grounded atabout its mid-point. Since the direct current potential of the amplifierpanel which is connected to ground is correct for supplying the platevoltage of the first tube and for connecting the cathode of the secondtube, just one battery filter of .the type used by the circuit of Fig. 2is required for .both the first two stages.

Since the initial stages of the amplifier of Fig. 3 have their spacecurrent circuits connected in series, the anode of device 4| is-directlyconnected to the input control grid of device 42 and the resistance -I3is suflicient to provide the desired biasing potential for the controlgrid of device 42. The path of the direct current for the first twostages in series is from the positive terminal I of the space currentpower supply P through the filter retard coilrz for the first two stagesto the screen grid of tube 42 and through retard coil 30 to its plate,then through the space of tube 42 to its cathode and suppressor grid,thence to the plate of tube 4| through retard coil 3 and resistance l3to its screen grid, then through the space of tube 4| to its cathode andsuppressor grid, through cathode resistance 4, thence through the filterretard coil 8 and back to the negative terminal 6 of power supply P.Most of the alternating current, generated in tube 4| by condenser 5,and that which is not is shunted around the filament battery bycondenser 9. The alternating current generated in tube 42 is bypassedaround the plate battery by condenser E. For a more detailed discussionof an amplifier having its first two tubes in series, reference may bemade to United States patent to J. B. Maggio, No. 2,254,377, issuedSept. 2, 1941, for Repeater station.

The additional circuit elements 62, 64, 65, 66 and those contained in 68are components of a feedback circuit used in the amplifier of Fig. 3 andform no part of this invention. The additional wiring ll between thefilter and amplifiers is necessary to connect the mid-point of thisfilter with amplifier ground l2 and battery ground it! because of thespecial space current voltage requirements of this amplifier asdiscussed above. Additional wiring i4 is provided to connect resistance12 which is common to all output stages to supply them with a grid bias.Resistance 61 is also associated with the feedback circuit of thisamplifier.

The fact that this invention has been described with reference toamplifiers having only three stages as found in some telephone repeaterstations in no way limits its application. The scope of this inventionis defined by the following claims.

What is claimed is:

1. In a repeater station having a multiplicity of lines passing throughit, each line conveying electrical signals, an amplifier interposed ineach line, said amplifier having a plurality of stages,.

each stage comprising an electron discharge device for raising the levelof said signals, means connecting the space paths of all said electrondischarge devices which raise said signals to the same level in parallelwith each other as a group, one common source of space current for allof said groups, and a wave filter individual to each of said groups,each such filter interposed between its group and said common sourcewhereby crosstalk between said amplifiers is minimized.

2. In combination, a plurality of means for altering in a desired waysignal currents passing through them, said means composed of severalconnected circuits arranged in stages, the various stages havingelectron discharge devices, the level of the signal currents diiIeringfrom stage to stage, all stages wi' 1. signal currents at the same levelhaving their space current circuits connected in parallel as a group, alowpass filter for each such group, and a common source of space currentsupply, each said filter inserted between its group and said spacecurrent source whereby amplifier crosstalk is appreciably reduced.

is by-passed 1 circuits of all stages having signal currents of the sameintensity being connected in parallel as a group, a common source ofspace current supply, and a low-pass filter comprising a series coilterminated in a shunt condenser inserted between each such group andsaid source of space current, each said shunt condenser connecteddirectly across each said group to suppress intra stage to stage, allstages of said amplifiers in which signal currents are at the same levelhaving their space current circuits connected in parallel, a singlesource of space current supply, and a low-pass wave filter insertedbetween each parallel circuit so formed and said space current source,said low-pass filter having a high inductance series arm and a highcapacity shunt arm, said shunt arm connected adjacent to said amplifier,said series arm preventing signal cur- I rents from passing through thebattery thereby preventing crosstalk between stages of different leveland said shunt arm presenting a low impedance to signal currents therebypreventing crosstalk between stages of the same level.

5. In combination, a group of amplifiers having a plurality of stages,signal currents passing through all such amplifiers simultaneously, thespace current circuits of all stages with signal currents of the samelevel b'eing connectedin parallel, a single source of space currentsupply, and a separate low-pass filter inserted between said source andeach said parallel circuit, said filters preventing the currentsgenerated in any stage of any amplifier from crosstalking into any stageof any amplifier.

6. In a telephone repeater station, a multiplicity of lines passingtherethrough, all said lines carrying signal currents simultaneously, amultistage amplifier in tandem with each line for raising the level ofsaid signal currents in stages, each amplifier being mounted on aseparate panel and having its individual panel ground return circuit, asingle space current battery for all of said amplifiers, said batteryhaving its ground at a point intermediate its positive and negativeterminals, all space current circuits of stages at the same level beingconnected in parallel as a group, and a filter with a balanced to groundconfiguration inserted between each of said groups and the positive andnegative ends of said battery, the mid-point of each of said filters '7.The combination of claim 6 in which at leasttwo of the stages havetheirspace current circuits connected in series, all series circuits soformed being connected inparallel as a group and each such groupconnected to space current battery through a respective one of saidfilters.

JAMESO. EDSON.

